Heretofore, most of roll-screens have utilized an elastically restoring force of a coil spring for winding.
As apparatus for utilizing the elastically restoring force of the coil spring, there have been commerically available an apparatus containing centrifugal clutch for transmitting the elastic force when a rotation rate reaches a predetermined level or other type of clutches, or apparatus directly connected to the coil spring. With the latter apparatus, the winding rate depends on a circumferential velocity of a winding pipe, namely a diameter of the pipe and on a weight (or a length) of a portion of the roll-screen which has been drawn out of the pipe. Thus, upon start of winding a smallest circumference of the pipe and a heaviest weight of the screen make the winding rate slower. As the screen is continuously wound onto the pipe, the circuference of the pipe becomes larger while the weight of the screen is decreased, resulting in the increased winding rate. Thus, toward the end of the winding, the rate of winding the remaining screen is accelerated and becomes very fast. As a result, on the finish of the winding, the rollscreen itself and fittings for the roll-screen may be damaged or adversely affected by an impact of collision.
There have been commercially available various types of clutches for overcoming the disadvantage as described above, such as a centrifugal clutch which may utilize an elastic force of a coil spring as a winding force upon a predetermined level of the rotation rate and is very convenient in handling. In other words, the centrifugal clutch includes a mechanism which does not convert the elastic force to the winding force until the predetermined rotation rate by using a coil spring of weak elastic force. In general, the weak elastic force of the coil spring can not initiate the winding of the roll-screen at the start requiring a considerable energy. The centrifugal clutch does not transmit the winding force until the predetermined rotation rate and actuates a clutch upon reaching the predetermined rotation for converting the elastic force (or rotational force) into the winding force, but may overcome the stationary energy at the start of winding the roll-screen. Thus the centrifugal clutch has no problems in winding the roll-screen at the start, but has a disadvantage in that when the winding is discontinued in the half way and subsequently restarted the elastic force is weakened correspondingly so that the winding can no longer be done. If the coil spring of a stronger elastic force is used for overcoming such disadvantage, the winding force at the start is correspondingly strong, so that the damage of the roll-screen may frequently occur at the start of the winding.